FIG. 1 shows the typical architecture of a digital pixel sensor. There is an ADC in each pixel 10 and a memory 12 external to the image plane which stores the image data. The common architecture includes a comparator 14 inside each pixel 10. One input to the comparator 14 is the photodiode voltage, the other input is a reference line Vramp 16 common to all the pixels 10.
The rate of change of the voltage on the photodiode is proportional to the amount of light impinging on the pixel. On a graph of voltage against time, an increase in incident light will result in a steeper slope. By measuring the time it takes for a given swing (ΔV) on the pixel, the amount of light can be determined. This is illustrated in FIG. 2.
FIG. 3 shows a typical comparator structure. Ideally, the transistor M3 will be switched on when Vphotodiode=Vramp. However, due to manufacturing tolerances transistors do not have the same threshold voltage (Vt), i.e., Vt_m2≠Vt_m3. This can be re-written as Vt_m2=Vt_m3+δVt3 or Vt_2−δVt3=Vt_m3. There is a voltage offset Vos 18 (FIG. 4) required between the two inputs to make the output change states.
FIG. 4 illustrates the operation of a comparator 14. With an ideal comparator, the output changes when voltages applied at the comparator's inputs, VinM and VinP, are equal. Adding the input offset voltage 18 as shown in FIG. 4, the output will change when VinM−Vos=VinP. The Vos will be different from pixel to pixel, even on the same device.
With the digital pixel architecture of FIG. 1, this offset voltage causes a shift to the reference level, and an offset is added to the digital output. Take two pixels, with Vos1 and Vos2, and all the pixels have a common reference Vref connected to the comparators' VinP inputs. The VinM inputs are connected to the pixel photodiodes.
Pixel 1's comparator will change when VinM1−Vos1=VinP, similarly for pixel 2. Therefore, Vphotodiode1−Vos1=Vref, and Vphotodiode2−Vos2=Vref. This can be re-written, assuming Vref1=Vref+Vos1 and Vref2=Vref+Vos2. Pixel 1's comparator will therefore change when Vphotodiode1=Vref1, and pixel 2's comparator will therefore change when Vphotodiode2=Vref2.
The input offset voltage appears to shift the reference level between pixels. This effect and its results can be seen in FIG. 5. This offset can detract from the quality of image obtained by the sensor, and therefore, methods have been sought to provide offset cancellation.
Known methods of analog cancellation require the storage of a reset voltage and the comparator offset on the photodiode of the pixel. This technique requires an increase in the actual voltage swing inside the pixel. To implement this, either the power supply needs to be increased or the operating range of the pixel is reduced.
More modern technologies have a lower operating voltage (e.g., 3.3V used to be common, 1.8V is now standard). The use of more modern technologies is needed to stay cost competitive, so the pixel needs to be re-designed to operate from lower voltages. Reducing the operating range is to be avoided since this increases the effect of noise sources, both internal (e.g., transistor thermal noise) and external (e.g., photon shot noise). There is a need to cancel the offset and also to account for fixed pattern noise (FPN), without either reducing the voltage swing on the pixel or increasing the voltage supply.
Digital cancellation can also be achieved by dark frame cancellation. This involves taking a dark reference frame and subtracting this from the image. In known digital cancellation techniques, the dark reference frame is taken with the same exposure (integration time) as the main image, but no light is impinged on the sensor. This may be achieved either using a shutter (mechanical or LCD) or by turning off the scene illumination.
On low performance systems, the black reference frame is acquired rarely. This leads to problems since the environment (especially the sensor's temperature) will change, such as causing differences in the offsets. A better technique is to acquire a dark reference frame shortly before image acquisition. The temperature of the sensor will then be similar for the dark reference frame and image frame, and the dark reference frame will be more accurate.
This technique is an effective method for removing dark current, since the integration time is the same, for both the image and dark reference frames. However, there are some disadvantages with dark frame cancellation. It requires acquiring 2 full frames for each output image. The acquired frame rate is therefore halved. As two images are subtracted, the random noise component is increased by √2. Reset noise (kTC noise) is not cancelled since it will be different for each frame.
For lower speed applications, dark current is a significant noise source and the above technique is very effective. For higher speed applications, dark current is much lower and of less concern and halving the frame-rate is often impractical.
Another cancellation technique is correlated double sampling (CDS). This involves taking a measurement immediately after reset and again at the end of exposure (i.e., readout). Subtracting the two values removes reset noise (kTC) and also offset. However, implementation of CDS at the pixel requires some extra space for the storage node, which reduces the amount of area in each pixel available for light collection (i.e., fill-factor), and the sensor's sensitivity is reduced.